polymer compositecrystal growth

Crystal Growth • How do single crystals differ from polycrystalline samples? Single crystal specimens maintain translational symmetry over macroscopic distances (crystal dimensions are typically 0.1 mm – 10 cm). • Why would one go to the effort of growing a single crystal? -Structure determination and intrinsic property measurements are preferably, sometimes exclusively, carried out on single crystals. -For certain applications, most notably those which rely on optical and/or electronic properties (laser crystals, semiconductors, etc.), single crystals are necessary. Estimated shares of world crystal production in 1999. (Reprinted from H. J. Scheel, J. Cryst. Growth 211(2000) 1–12. • What factors control the size and purity of single crystals? -Nucleation and Growth. If nucleation rates are slow and growth is rapid, large crystals will result. On the other hand, if nucleation is rapid, relative to growth, small crystals or even polycrystalline samples will result. • What can be done to increase the growth rates? -In order to attain the rapid growth rates needed to grow macroscopic crystals, diffusion coefficients must be large. Hence, crystal growth typically occurs via formation of a solid from another state of matter : (a) Liquid (Melt) àSolid (Freezing) (b) Gas (Vapor) à Solid (Condensation) (c) Solution à Solid (Precipitation) • It should be noted that defect concentrations tend to increase as the growth rate increases. Consequently the highest quality crystals need to be grown slowly. • What can be done to limit the number of nucleation sites? Several techniques are used separately or in combination to induce nucleation of the solid phase at a slow and controlled rate : (a) Slow Cooling of Melts (b) Temperature Gradients (c) Introduction of Seed Crystals Slow cooling of the melt • With congruently melting materials (those which maintain the same composition on melting), one simply melts a mixture of the desired composition then cools slowly (typically 2-10 °C/h) through the melting point. • More difficult with incongruently melting materials, knowledge of the phase diagram is needed. • Very often, the phase diagram is not known. Consequently, there is no guarantee that crystals will have the intended stoichiometry. • Molten salt fluxes are often used to facilitate crystal growth in systems where melting points are very high and/or incongruent melting occurs. • Crystals grown in this way are often rather small. Thus, this method is frequently used in research, but usually not appropriate for applications where large crystals are needed. Congruent and Incongruent Melting in Binary and Ternary Systems • The thermal behavior of intermediate compounds is of three basic types: congruent melting, incongruent melting, or dissociation. • An intermediate compound is a combination of the two end members of a binary or ternary phase diagram that forms a different component between the two solids. • Congruency of melting is important in the determination of phase analysis diagrams and in drawing crystallization paths. Congruent Melting • Binary Systems – In binary systems, compounds are composed of various ratios of the two end members (A & B), or the basic components of the system. – These end members are assumed to melt congruently. – The intermediate compound AB 2 melts congruently, because at some temperature (the top of the AB 2 phase boundary line) it coexists with a liquid of the same composition. Incongruent Melting • Binary Systems – The end components in this binary phase diagram also melt congruently. – The intermediate compound in this diagram (XY 2 ) however is incongruently melting. – Incongruent melting is the temperature at which one solid phase transforms to another solid phase and a liquid phase both of different chemical compositions than the original composition. – This can be seen in this diagram as XY 2 melts to Y and liquid. Multiple Incongruent Melting Regions • Binary Systems – This diagram shows many different intermediate compounds (Q,R,&S) that melt incongruently. – Each of these intermediate compounds melts to a liquid and a solid of a different composition. The Development of Crystal Growth Technology The Development of Crystal Growth Technology HANS J. SCHEEL SCHEEL CONSULTING, CH-8808 Pfaeffikon SZ, Switzerland Figure 1.1 Stages of flame-fusion (Verneuil) growth of ruby, schematic: (a) formation of sinter cone and central melt droplet onto alumina rod, (b) growth of the neck by adjustment of powder supply and the hydrogen- oxygen flame, (c) Increase of the diameter without overflow of the molten cap for the growth of the single-crystal boule. (Reprinted from H. J. Scheel, J. Cryst. Growth 211(2000) 1–12) [...]... Examples : Growth of Fe3O4 crystals Fe3O4 (s) + 8HCl (g) à FeCl2 (g) + FeCl3 (g) + 4H2O (g) (Endothermic) • Growth of ZrNCl crystals ZrNCl (s) + 3HCl (g) è ZrCl4 (g) + NH3 (g) (Exothermic) • Growth of Ca2SnO4 crystals SnO2 (s) + CO à SnO (g) + CO2 (g) SnO (g) + CO2 (g) + 2CaO (s) à Ca2SnO4 (s) + CO (g) • Chemical Vapor Transport is a good method of growing high quality crystals from powders However, growth. ..Modification of Verneuil’ s principles of nucleation control and increasing crystal diameters in other crystalgrowth techniques (Reprinted from H J Scheel, J Cryst Growth 211(2000) 1– 12 Figure 1 The Stockbarger-type furnace Zone Melting • A polycrystalline specimen is prepared, typically in the shape of a cylinder and placed into a crucible,... for industrial applications Laser Heated Pedestal Growth (LHPG) 雷射加熱提拉生長法 The LHPG technique is derived from the zone melting method and capable of producing a large variety of crystal fibers In practice, one can grow fibers approximately 20~300um in cross section with this technique Nonlinear laser crystal as a blue converter: laser heated pedestal growth, spectroscopic properties and second harmonic... diagram we chose to pull the fibres at rates ranging between 20 and 33 mm h- 1 At the end of the growth, the fibres were annealed at 900 0C for 8 h under an oxygen flow Cross section of an a-axis oriented KLN fibre: (a) experimental cross section of a K3Li2- xNb5+xO15+2x, x = 0.24/ fibre: (b) idealized growth symmetry Surface morphologies of a K3Li2- xNb5+xO15+2x, x = 0.24/ fibre: (a) view of c-plane;... melt (the surrounding atmosphere is cooler than the melt) • Decreasing the speed with which the crystal is pulled from the melt, increases the quality of the crystals (fewer defects) but decreases the growth rate • The advantage of the Czochralski method is that large single crystals can be grown, thus it used extensively in the semiconductor industry • In general this method is not suitable for incongruently... crystal is heated to the melting point, and the rest of the cylinder is slowly pulled through the hot zone • Zone melting setups are modifications of either the Bridgman or Stockbarger methods of crystal growth • Bridgman Hot zone moves, crucible stationary Stockbarger Crucible moves, hot zone stationary Czochralski Method • A seed crystal is attached to a rod, which is rotated slowly • The seed crystal . H. J. Scheel, J. Cryst. Growth 211(2000) 1–12. • What factors control the size and purity of single crystals? -Nucleation and Growth. If nucleation rates are slow and growth is rapid, large. is rapid, relative to growth, small crystals or even polycrystalline samples will result. • What can be done to increase the growth rates? -In order to attain the rapid growth rates needed to. Crystal Growth Technology The Development of Crystal Growth Technology HANS J. SCHEEL SCHEEL CONSULTING, CH-8808 Pfaeffikon SZ, Switzerland Figure 1.1 Stages of flame-fusion (Verneuil) growth